Despite remarkable advances in the treatment of HIV infection, the ability to eradicate the virus from an infected individual still evades us. Thus the current focus of treatment remains maximal suppression of viral replication. Sensitive HIV-1 RNA assays have shown that viral replication persists even in those patients considered to have suppressed virus (HIV-1 RNA < 20 copies/ml). Gut-associated lymphatic tissue (GALT) and CD4 cells appear to be the principal anatomic site and cellular source of persistent virus production, by ongoing reactivation from a latently infected pool and low level replication before, during, and after treatment interruption. Nucleos(t)ide reverse transcriptase inhibitors (NRTIs) constitute the backbone of virtually all combination antiretroviral regimens used today. No data have been published describing NRTI-TP concentrations in the tissues where replication predominately occurs, the lymphoid tissues like secondary lymph nodes and gait. Therefore, investigations of NRTI-TP and intracellular PI concentrations in these protected sites are paramount to uncovering potential mechanisms for persistent viral replication in the face of seemingly potent antiretroviral treatment. The hypothesis for this project Intracellular Compartmental Kinetics of HIV Drugs is: Cryptic replication is a consequence of inadequate concentration of drug in cells of GALT and LN. Four Specific Aims are proposed. 1: To define and compare NRTI-TP and PI concentrations in peripheral blood mononuclear cells (PBMCs), and GALT-associated and inguinal node CD4+ T cells. 2: To relate NRTI-TP and PI concentrations in PBMCs, and GALT-associated and inguinal node CD4+ T cells with inhibition of viral replication in that compartment. 3: To examine biological mechanisms for differing NRTI-TP and PI concentrations among PBMCs, and GALT-associated and inguinal node CD4+ T cells with a focus on cell activation and drug transporter status. 4: To develop a pharmacokinetic-pharmacodynamic model that quantifies NRTI-TP and PI concentration kinetics in these compartments, biological factors that influence these concentrations, and relationship to clinical response. The long term goal of this work is to determine the pharmacologic contribution to persistent viral replication and provide a scientific basis for investigations of whether effective antiretroviral therapy may require the use NRTIs that are preferentially phosphorylated in different compartments.